In 12.04 ext4 df -h shows different space usage compared to 11_04

ext4 FS shows 3% of space used in df -h ouptut on 12_04 compared to the same mount on 11_04 ; Is this pertaining to the latest kernel? Cant make out any other differences as the 'reserved block' is tuned already to 0

$ lsb_release -cr

Release: 12.04 Codename: precise

df -hP |grep media

/dev/sda3 21G 430M 20G 3% /media/MAIL

mount|grep -i mail
/dev/sda3 on /media/MAIL type ext4 (rw,nosuid,nodev,uhelper=udisks)

$ sudo tune2fs -l /dev/sda3 |grep 'Reserved block count'

Reserved block count: 0

***NO hidden files or directories

$ sudo du -ah *;pwd

16K lost+found /media/MAIL

-----------it shows different in my natty ---------------
$ df -hP|grep -i mail;lsb_release -cr
/dev/sda3 20G 172M 20G 1% /media/MAIL
Release: 11.04
Codename: natty
$ uname -r
2.6.38-14-generic
-------------------Any kernel bugs?-------------------------